1. Field of the Invention
The present invention generally relates to a deflection and high voltage circuit for a Cathode-Ray Tube (hereinafter, referred to as CRT), and more specifically to a deflection and high voltage circuit for display systems using CRT(s), for deflection and generation of a high tension anode voltage for the CRT.
2. Description of Background Information
Display devices including a CRT or CRTs, such as television systems, projection type television systems, etc., generally have such a structure that a deflection current is supplied to a horizontal yoke coil of the CRT, and a high voltage to be connected to the anode of the CRT is generated.
An example of conventional deflection and high voltage circuit is shown in FIG. 1. As shown, the circuit is constructed such that a horizontal driving signal is supplied to the base of a horizontal power transistor 1. First and second parallel circuits 2 and 3 forming a series circuit are connected between the collector and the emitter, which is grounded, of the horizontal power transistor 1. The first parallel circuit 2 is constituted by a damper diode 11, a damper capacitor 12 of a deflection retrace capacitor, a horizontal deflection winding 13, and an AC coupling capacitor 14 called S-curve correction capacitor. Similarly, the second parallel circuit 3 is constituted by a damper diode 16, a modulation retrace capacitor 17, a modulator inductor 18, and a modulator capacitor 19.
A junction between the modulator inductor 18 and the modulator capacitor 19 is connected, via a resistor 21, to the collector of a transistor 22 whose emitter is grounded and a horizontal amplitude correction circuit 9 is connected to the base of the transistor 22.
The primary (primary winding) of a flyback transformer 5 is connected, at its one end, to a line to which the collector of the horizontal output transistor 1 is connected, and a voltage supply circuit 6 is connected to the other end of the primary of the flyback transformer 5. The voltage supply circuit 6 has a series connection type constant voltage circuit for providing a power voltage +B, constituted by a transistor 23, a Zener diode 24 and a resistor 25. The output voltage of the series-connection type constant voltage circuit is connected via a resistor 27 and a diode 28 connected to the resistor 27 at its anode, through which the voltage flows in the normal direction, to the other end of the primary 5a. The output voltage appearing at the secondary winding of the flyback transformer 5 is rectified by a rectifying diode 7 and in turn supplied to the anode of the CRT (not shown).
A bleeder resistor 8 is connected, as a voltage dividing resistor, to the cathode of the diode 7. The output of the bleeder resistor 8 is connected to the horizontal amplitude correction circuit 9 including a buffer amplifier 30, and an inverting amplifier 31. A variable resistor 33 to adjust the horizontal size is connected, via a resistor 32, to the junction between the buffer amplifier 30 and the inverting amplifier 33, and a parabolic voltage Vp repeated at every field is connected via a resistor 34 for the purpose of the compensation of the pincushion distortion.
In the circuit structured as described above, the power voltage +B is produced as a DC voltage appearing at the emitter of the transistor 23 whose level is determined by the Zener voltage of the Zener diode 24. The power voltage +B produced in this way is supplied to the collector line of the horizontal output transistor 1 through the resistor 27, diode 28 and the primary 5a of the flyback transformer 5. When the horizontal output transistor 1 is switched off, charging of the capacitors 12, 17, 14 and 19 takes place.
When the horizontal output transistor 1 is turned on by the horizontal drive signal at a high level, currents flow from the capacitors 14 and 19 to the deflection winding 13 and the dummy winding 18 respectively, and the magnitude of the currents increases linearly. When the horizontal drive signal is at a low level to turn off the horizontal transistor 1, the electromagnetic energy accumulated in the deflection winding 13 and the dummy winding 18 is released to charge the retrace capacitor 12 and the modulation retrace capacitor 17 so that the voltage level at the terminal of the deflection coil goes up. This voltage is raised by the flyback transformer 5 as a horizontal flyback pulse signal, and the resultant high voltage is supplied, via a rectifying diode 7, to the anode of the CRT.
After the charging of the above-mentioned electromagnetic energy to the resonance capacitors 12 and 17, no current flows through the windings 13 and 18. Subsequently, the electrostatic energy stored in the retrace capacitor 12 and the modulation retrace capacitor 17 is discharged toward the deflection winding 13 and the dummy winding 18, so that a current of an inverse direction flows in the deflection winding 13 and the dummy winding 18. When the discharge of the resonance capacitors 12 and 17 is completed, the energy stored in the windings 13 and 18 is discharged via the damper diodes 11 and 16 respectively, and the charging of the capacitors 14 ad 19 will take place. When the transistor 1 is again switched on, the above-described series of operations repeat, so that the high voltage is produced and a horizontal deflection current of a sawtooth waveform flows through the deflection winding 13 at the same time.
The output voltage of the diode 7 is smoothed out by the capacitance of the anode of the CRT. The voltage applied to the anode of the CRT, which is smoothed out, is then voltage divided by the bleeder resistor 8, and supplied to the base of the transistor 22 via the buffer amplifier 30 and the inverting amplifier 31. Then, the transistor 22 enters into an active state depending on the potential level at the base thereof, and it operates as a variable load to vary the voltage V2 across the terminals of the capacitor 19. Since the parabolic voltage Vp is superimposed on the inverting amplifier 31, the active state of the transistor not only varies in response to the anode voltage, but also varies repetitively at every field period in response to the parabolic voltage Vp.
With this provision, the voltage V2 across the terminals is varied in response to the parabolic voltage Vp, to compensate for the pincushion distortion. The voltage V2 across the terminals is further varied relatively in response to the resistance of the variable resistor 33, so that the adjustment of the horizontal size is performed.
Since no DC voltage is produced across the terminals of the deflection winding 13 and the dummy winding 18 respectively functioning as an inductance, both the voltage V1 across the terminals of the capacitor 14 and the voltage V2 across the terminals of the capacitor 19 become DC voltages whose levels are determined by the impedance of each circuit. The sum V1+V2 of these voltages V1 and V2 is equal to the collector voltage Vc of the transistor 1.
Since the average value of the collector voltage Vc of the transistor 1 is equal to the constant voltage +B1 (the terminal voltage of the primary 5a of the flyback transformer), the collector voltage Vc does not vary at the field frequency. Furthermore, since the sum of the voltages V1 and V2 is equal to the voltage +B, the voltage V1 is rendered to vary at the field frequency in response to the variation of the voltage V2. In this state, the direction of the variation is opposite to that of the variation of the voltage V2.
As described above, if the voltage +B is maintained constant, the collector voltage Vc of the transistor 1 does not vary at the field frequency, so that the voltage applied to the primary 5a of the flyback transformer 5 will have no change at the field frequency. As a result, the deflection current flowing through the deflection winding 13 does not vary at the field frequency because the voltage V1 is maintained constant. The high voltage generated at the secondary winding 5b of the flyback transformer 5 also will not vary at the field frequency.
However, in the case of the conventional deflection and high voltage generation circuit mentioned above, there has been a drawback that fluctuation of the anode voltage is caused by the variation of the luminance signal level, so that the luminance level and the focusing of the CRT image cannot be obtained properly. More specifically, when the luminance signal level is high, it will cause the reduction of the anode voltage, which in turn will result in a broader horizontal size and a larger focus point. For the reason mentioned above, there has been a problem that proper luminance level and focusing performance cannot be obtained with the conventional deflection and high voltage circuit.